1. Field of the Invention
The present invention relates to a polymer electrolyte membrane, a method for producing the same, and a proton conductivity evaluation method for a polymer electrolyte membrane.
2. Related Background Art
In the past, perfluorosulfonates such as Nafions (E. I. Du Pont, registered trademark) that have proton conductivity have been known for their excellent fuel cell properties as polymer electrolytes to be used in solid polymer fuel cells.
However, because prior art polymer electrolytes are very difficult to use at high temperatures and are very expensive, the development of a polymer electrolyte membrane that can overcome these disadvantages has been quite active. In such development polymer electrolytes wherein a sulfonic acid group is inserted into an aromatic polyether are considered to have high potential because of their superior heat resistance and excellent membrane strength. For example, the polymer electrolyte membranes of a sulfonated polyether ketone system (Japanese Patent Application Laid-open No. H11-502249) and a sulfonated polyether sulfone system (Japanese Patent Application Laid-open Nos. H10-045913 and H10-021943), etc., have been proposed as such a polymer electrolyte.
Moreover, to obtain high power output a polymer electrolyte membrane for a fuel cell must have high proton conductivity. In general, increasing the phase that has an ionic functional group, such as a sulfonic acid group and the like, is known as a method to increase the proton conductivity of a polymer electrolyte membrane. Among these, a polymer electrolyte membrane having a microphase-separated structure is known to have high proton conductivity and to be affected little by humidity (Japanese Patent Application Laid-open No. 2003-031232).